Stout little X is our emissary to the unknown. When Wilhelm Konrad Roentgen discovered penetrating radiations of an unknown nature on Nov. 8, 1895, he called them X-rays.
As it turned out, the mysterious rays were not so mysterious as he at first imagined; they are of the very same nature as the rays that enter our eyes from the Sun, only of a shorter wavelength. Roentgen had stumbled into a previously unexplored part of the electromagnetic spectrum.
We live in a sea of electromagnetic waves, from the very long to the very short: AM radio, FM radio, television, radar, microwaves, radiant heating, visible light, ultraviolet, X-rays, gamma rays, cosmic rays. The wavelength (distance from crest to crest) of a typical radio wave might be as long as a city block. A billion X-ray waves can fit across the head of a pin.
What is it that is waving? Electric and magnetic fields. What are electric and magnetic fields? The things that wave.
That didn't help much. Let's try again.
What are electric and magnetic fields? The things that physicists represent in their equations by the letters E and B. What are E and B? The electric and magnetic fields.
Still unsatisfied? That's because you want to see or touch these waves, the way you see and touch waves in water or vibrating piano strings. Can't be done. Electromagnetic waves are electromagnetic waves -- and that's that.
But they are undeniably real. Turn on your radio. Thaw the hamburger in your microwave. Look at those CT-scan images my doctor ordered last week. Never mind that electromagnetic waves seem spookily immaterial, evanescent, hard to describe in familiar terms. Our equations precisely describe their behaviors. X goes forth in the world as respectable E and B.
The most familiar electromagnetic waves are visible light. Ten or twenty thousand light waves would fit across your thumbnail. They are focused by the lens of the eye and detected by the retina. We were born with electromagnetic wave detectors on either side of our nose.
Our bodies are less dramatically sensitive to waves a little shorter than the visible, called ultraviolet. These can change cells in our skin and give us a tan or skin cancer. We are also sensitive to waves a little longer than visible, called infrared. We feel them as heat.
The rest of the spectrum, stretching away toward the very long and the very short, was X-territory -- until the late-19th century.
We followed X into the unknown. Down the spectrum to longer and longer wavelengths, and up the spectrum to shorter and shorter wavelengths. We learned how to produce each kind of wave, turn it to our purpose. We learned how to detect those waves falling upon the Earth from space, and used what we detected to discover more about the universe.
We are creatures of X. We love our mysteries. When Roentgen announced his discovery, the news spread like wildfire. Public demonstrations sprang up everywhere. "Wondrous rays." "See the bones in your hand." "Count the coins within your purse." Now, Roentgen's rays have now been assigned to their appropriate place in the electromagnetic spectrum. They are as well-understood as the colors of the rainbow. Maybe it's time to give them a name less suggestive of mystery: Roentgen-rays, perhaps, or more simply, R-rays.